AC power line voltages of various magnitudes are conventionally used as a source of power to drive other circuits. AC power line voltages are available at every house, business and in most geographical areas for powering electrical equipment. AC voltages generated by stationary or mobile generators, such as utility generators, are used to generate AC voltages for powering industrial machines, aircraft, etc. In certain instances, inverters are used to convert DC voltages into corresponding AC voltages to provide a source of AC energy.
AC power line voltages in the United States are characterized by a sine wave having an RMS value of about 110 volts and a peak value of about 155 volts. The power line voltage is susceptible to transient voltages imposed thereon. The transient voltages can arise from lightning strikes to the power lines, inductive circuits such as electric motors starting or stopping, and the switching of electric utility lines at substations. Transients on the order of a thousand volts, or more, can be imposed on the AC power lines as a result of the foregoing.
The AC voltages that are available for powering electrical machines and circuits are typically converted to another type of electrical energy. For example, AC voltages are often converted to DC voltages for powering electronic circuits such as computers, radios, televisions, household appliances, fax machines, telecommunications equipment and circuits, control systems, etc. These and numerous other types of electrical equipment utilize semiconductor circuits for controlling electrical signals to accomplish desired functions. Semiconductor circuits are susceptible to damage due to transient voltages that exceed the safe operating voltage of the semiconductor devices. Accordingly, there are numerous overvoltage protection circuits that have been designed to protect semiconductor circuits from the adverse affects of overvoltages. Transistors, silicon controlled rectifiers (SCRs), triacs, other thyristors, metal oxide varistors (MOVs), Zener diodes and many other types of semiconductor devices are used in overvoltage protection circuits. In general, overvoltage protection circuits are threshold sensitive so that when a transient voltage exceeds the threshold, the overvoltage protection circuit triggers or otherwise conducts to clamp the overvoltage to a safe level and shunt the resulting current away from the circuits to be protected. Many overvoltage protection circuits are not designed to carry large sustained overcurrents resulting from an overvoltage, but rather rely on upstream fuses or circuit breakers to operate and isolate the circuits from the overvoltage and overcurrent conditions.
As noted above, there are many types of solid state devices that have been used in overvoltage protection circuits. The MOV is a popular device for clipping voltage transients above specified thresholds. While the MOV is well adapted to absorb high levels of overvoltage energy, such devices are not responsive to high speed transients, they degrade over time, and they do not have well defined voltage thresholds. Newer semiconductor devices have been developed that are responsive to high speed transients and have well defined breakover voltages. Such devices are known as SIDACtor® overvoltage protection devices and are obtainable from Teccor Electronics of Irving, Tex. The SIDACtor overvoltage protection devices are constructed as avalanche devices and thus exhibit negative resistance characteristics. U.S. Pat. No. 5,479,031 by Webb, et al. discloses the structural and operational features of SIDACtor overvoltage protection devices. It has been known to utilize a series combination of a SIDACtor overvoltage protection device and an MOV to provide transient protection to AC lines.
While overvoltage protection circuits may be handy, optional or required, it is generally a requirement that such circuits be simple and cost effective. Another design criteria is that such circuits be fast acting and transparent to the operation of the circuit to be protected. Another design consideration is for such overvoltage protection circuits to be nondestructive and less prone to degradation after one or more operations. Another consideration is that the overvoltage protection circuits do not trigger on the peak amplitude of the AC voltage itself, but only on voltage transients superimposed on the AC waveform. Moreover, such overvoltage protection circuits should be designed to return to the non-conductive state when the transient is no longer present, even though the AC voltage remains present. If the overvoltage protection circuit remains conductive after the transient has disappeared, the overvoltage protection circuit would become overheated and destroyed due to the energy available by the AC power source. In this instance, the overvoltage protection circuits would have to be constructed of unnecessarily heavy duty components, thereby rendering the circuits cost prohibitive.
As an example to the foregoing, if a thyristor device were to be connected directly across a 110 volt AC line and designed to provide a breakover or threshold voltage of 200 volts, such thyristor device would remain transparent to the other downstream circuits connected to the AC line, for all AC line voltages that do not exceed the threshold voltage of 200 volts. In the event that a transient voltage exceeding the amplitude of the threshold voltage appears on the AC line, the thyristor device would be triggered into conduction to clamp the AC line to voltages that do not exceed the safe operating voltage of the circuits coupled to the AC line. During the time of conduction, the thyristor device not only conducts the current resulting from the transient voltage but also conducts the full AC current that can be supplied by the AC line. This can be substantial energy even for the half cycle, or so, in which the thyristor device conducts. The surge current that can be supplied by the AC line, if supplied by a conventional household outlet can be 1,000 amps or more. The amount of energy that is available from the 110 volt line is generally much more than can be sustained by the thyristor for any significant period or time.
When the AC voltage passes through a zero-voltage level and the transient is no longer present, the thyristor device will automatically turn off as the current conducted by the thyristor device will be less than the holding current of the device. On the other hand, if the voltage on the AC line continues to exceed the threshold voltage of the thyristor device, such as when 220 volts has been inadvertently connected to the 110 volt line, or when a transient exists for each AC cycle, then the thyristor device will continue to conduct. In this instance, unless a circuit breaker operates, a fuse blows, or another overcurrent protection device operates, the thyristor device may be destroyed.
In order to provide safe operation to the various overvoltage protection circuits, a fuse is generally coupled in series with the hot AC conductor. Thus, rather than allowing the AC energy to be dissipated across the components of the overvoltage protection circuit, the overcurrent flowing in the AC line will blow the fuse to thereby isolate the down stream section of the AC line. The faster the overvoltage protection circuit functions in response to transients, the higher the possibility of nuisance firings and corresponding blowing of the fuse.
The threshold sensitive devices identified above cannot be coupled directly from the hot AC conductor to ground for the reasons noted. The placement of a power resistor in series with the threshold sensitive device is not practical as the resistance can limit the effectiveness of the threshold sensitive device.
From the foregoing, transient suppressors could be enhanced by a simple circuit that is sensitive to the overvoltage transient itself, but is not sensitive to the underlying AC line voltage.